1. Field of the Invention
The present invention relates to processing of semiconductor wafers and other electronic substrates such as slices of semiconductor silicon and other articles requiring a planar surface, and, more particularly, to an improved method and apparatus for polishing the wafers using the chemical-mechanical planarization (CMP) process and achieving high polishing rates and wafer planarity and uniformity while using smaller amounts of the chemical slurry than conventionally used in the CMP process.
2. Problem to be Solved
In the manufacture of integrated circuits wafer surface planarity is of extreme importance. Photolithographic processes are typically pushed close to the limit of resolution and it is essential that the wafer surface be highly planar so that the electromagnetic or other radiation used to create the integrated circuit may be accurately focused in a single level thus resulting in precise imaging over the entire surface of the wafer. Wavy, curved or wedge-shaped semiconductor disks result in lack of definition when, for example, a photosensitive resist is applied to the surface of a "non-planar" disk and exposed.
In order to achieve the degree of planarity required to produce ultra high density integrated circuits, chemical-mechanical planarization processes are now typically employed in the industry. In general, the chemical-mechanical planarization (CMP) process involves pressing a rotating semiconductor wafer or other such electronic component or other substrate against a moving polishing surface that is wetted with a chemically reactive, abrasive slurry. The slurries are usually either basic or acidic and generally contain alumina or silica particles. The polishing surface is typically a planar pad made of a relatively soft, porous (open pored) material such as blown polyurethane. The pad is usually mounted on a planar rotating turntable platen but may also be a rectilinear moving endless belt as is known in the art.
In general, for a rotating turntable the wafer is secured to a carrier plate (or wafer carrier) by vacuum or by a mounting medium such as an adhesive, with the wafer having a force load applied thereto through the carrier by a pressure plate so as to press the wafer into frictional contact with a polishing pad mounted on a rotating turntable. The carrier and pressure plate also rotate as the result of either the driving friction from the turntable or rotation drive means directly attached to the pressure plate. A typical way of securing and releasing the wafer is by the use of a vacuum head that includes a rigid perforated plate against which the wafer is drawn by applying a vacuum to a plenum lying above the perforated plate.
All chemical-mechanical polishing (CMP) processes are dependent on the ability of a polishing template, or pad, to transport the polishing medium, or slurry, to the substrate surface efficiently. This transport of slurry is inhibited by the gradual accumulation of polishing by-products on the template or pad surface. These by-products tend to fill the natural surface porosity of the pad as the polishing process continues over time, and this causes the polishing rate to decrease and the non-uniformity of the polishing process to increase.
Polish by-products may be partly removed by "flooding" the pad surface with additional slurry, although this is an expensive response to the basic problem, and it is not completely effective. For these reasons, the polishing process is expensive to control, especially in high-volume production applications. Due to a lack of polish-rate stability, it is difficult to predict the duration of a polishing process, and the polish times tend to increase on successively polished substrates unless the polishing surface is treated by specific means. These problems decrease product throughput, which is a major cost-driver in CMP.
Surface transport issues also affect the uniformity of the polishing across the substrate surface. Further, the buildup of polish by-products on the pad surface may also increase the incidence of physical defects on the wafer surface. Both of these latter problems (uniformity and defects) also increase production costs, because they may decrease product yields, another primary driver of process costs.
A current method of chemical slurry application involves dripping slurry onto the polishing pad through a tube so as to pool the slurry in the center of the pad. This method is generally inefficient to coat the pad and excess slurry is typically applied to maintain a fluid layer between the pad and the wafer. A fluid layer is considered necessary to achieve an acceptable polishing rate and polishing uniformity.
U.S. Pat No. 4,910,155 describes the basic CMP process and utilizes a retaining wall around the polishing pad and polishing table to retain a pool of slurry on the pad. U.S. Pat No. 5,403,228 discloses a technique for mounting multiple polishing pads to a platen in a CMP process. A seal of material impervious to the chemical action of the polishing slurry is disposed about the perimeter of the interface between the pads and when the pads are assembled the bead squashes and forms a seal and causes the periphery of the upper pad to curve upward creating a bowl-like reservoir for increasing the residence time of slurry on the face of the pad prior to overflowing the pad. U.S. Pat No. 3,342,652 shows a process for chemically polishing a semiconductor substrate and a slurry solution is applied to the surface of the pad in bursts as a stream forming a liquid layer between the cloth and the wafers to be polished. The solution is applied from a dispensing bottle and is applied tangentially to the wafer-plate assembly so as to provide maximum washing of the polishing cloth in order to remove waste etching products. U.S. Pat. No. 4,549,374 shows the use of a specially formulated abrasive slurry for polishing semiconductor wafers comprising montmorillonite clay in deionized water.
All the above patents are hereby incorporated by reference.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an apparatus, e.g., a CMP apparatus, for polishing semiconductor wafers and other workpieces using smaller amounts of the chemical or other slurry used to polish the workpiece while still maintaining the polishing rate and uniformity of the polished surface.
It is another object of the present invention to provide an improved method for polishing workpieces, e.g., semiconductor wafers, using such a polishing apparatus as the CMP system and using the improved CMP apparatus of the invention.
It is an additional object of the invention to provide a spray atomization device for use in a polishing apparatus for polishing electronic substrates such as wafers, e.g., using a CMP apparatus, and for use in polishing methods such as CMP methods to enhance the efficiencies and operation of the CMP apparatus and method.
It is a further object of the invention to provide planar workpieces, including semiconductor wafers, made using the improved method and apparatus of the invention.
Other objects and advantages of the present invention will be readily apparent from the following description.